1. Field of the Invention
The present invention relates to a universal motor. More specifically, the present invention discloses a universal motor that has reduced electromagnetic interference characteristics.
2. Description of the Prior Art
Universal motors are electric motors that, because of their ability to accept both AC and DC power sources, can be found quite readily in the market place, such as in cordless powered screwdrivers, blenders, etc. Universal motors are also in wide use in industry, as they have a simple, reliable design.
Please refer to FIG. 1. FIG. 1 is an exploded view diagram of a prior art universal motor 10. The universal motor 10 comprises a stator 20, a rotor 30 and end brackets 50 and 70. The rotor 30 has a shaft 32 onto which are mounted a commutator 34 and a rotor core 36. The rotor core 36 is wound with copper wire (not shown) to form many individual groups of windings. Each of these windings is electrically connected to a corresponding surface 35 on the commutator 34. The ends of the shaft 32 are rotatably fixed in bushings 52 and 72 of the end brackets 50 and 70, respectively. The end brackets 50 and 70 are firmly mounted onto the stator 20. Hence, the rotor core 36 rotates within the stator 20, and the commutator 34 rotates with the rotor core 36. The stator 20 also has many windings 22, which have leads 24 for delivering electrical power to the windings 22. The end bracket 50 comprises brush holders 60 for elastically holding brushes 62 in contact with the commutator 34. Electrical power is delivered by the brushes 62 to the commutator 34, and hence to the appropriate windings on the rotor core 36. Each brush holder 60 comprises a socket 61 into which a brush 62 is slidably disposed, a spring 64 for elastically pushing the brush 62 towards the commutator 34, and an external contact 66. The external contact 66 is electrically connected to the brush 62, and is used to deliver electrical power to the brush 62. Wires 68 lead off from the external contacts 66 to a power source (not shown).
As the universal motor 10 operates, the rotor 30 spins and the brushes 62 continuously make and break electrical contact with the surfaces 35 on the commutator 34. This constant making and breaking of contact leads to electrical arcing between the brushes 62 and the commutator 34. Such electrical arcing is a source of electromagnetic interference (EMI). To reduce the intensity of the EMI, the lead wires 68 first connect to a filter circuit (not shown), and the filter circuit is connected to the power source. Although the filter circuit does help to reduce some of the EMI, it cannot prevent all of the arcing. Furthermore, the long lead wires 68 to the filter circuit become a source of EMI, as they radiate like antennas.
It is therefore a primary objective of this invention to provide a universal motor that has a filter circuit on a printed circuit board, with the printed circuit board mounted over the commutator to serve as a shield against EMI caused by arcing between the commutator and the brushes.
The present invention, briefly summarized, discloses a universal motor with reduced electromagnetic interference (EMI) characteristics. The universal motor has a stator, a rotor, brushes, and a filter circuit on a printed circuit board (PCB). The rotor has a shaft, a commutator mounted on the shaft, and a rotor core mounted on the shaft adjacent to the commutator. The rotor core is electrically connected to the commutator, and rotates within the stator. The brushes deliver electrical power to the commutator, and they elastically contact the commutator. The filter circuit reduces the emission of electromagnetic radiation from the universal motor, and is electrically connected to the brushes. The PCB is mounted over the brushes. When electrical arcing occurs between the commutator and the brushes, the PCB acts as a shield to absorb a portion of the radiation emitted by the arcing so as to reduce the EMI characteristics of the universal motor.
It is an advantage of the present invention that the reduced EMI characteristics make the universal motor more convenient to use in environments where EMI effects are not well tolerated. Additionally, because the PCB is mounted directly over the commutator, no lengthy connecting wires are needed to connect the brushes to the filter circuit, and this further reduces the EMI characteristics of the present invention.